In a portable electronic device such as an information device, a display section is foldable with respect to a main body section, which makes it easy to carry the portable electronic around. In such an electronic device, the main body section and the display section are folded into a compact size when the electronic device is not used, and the display section is opened by being rotated with respect to the main body section when the electronic device is used, which makes it possible to identify the display section visually. Such a hinge device is disclosed in, for example, Japanese Laid-open Patent Publication No. 2011-33152.
FIG. 1A depicts an installation position of a hinge device 9 in a comparative technique, the installation position of the hinge device 9 in a notebook-size personal computer (hereinafter referred to as a notebook personal computer) 3 which is an electronic device, in a state in which the notebook personal computer 3 is opened. The notebook personal computer 3 includes a main body section 1 which is a first housing and a display section 2 which is a second housing. The display section 2 is openable and closeable with respect to the main body section 1 by using the hinge device 9 as a rotation center, and the main body section 1 and the display section 2 are connected to each other by two hinge devices 9.
FIG. 1B depicts the installation position of the hinge device 9 of the comparative technique, the installation position in the notebook personal computer 3, in a state in which the notebook personal computer 3 is closed. Moreover, in FIG. 1B, members forming the hinge device 9 of the comparative technique are depicted in a size corresponding to the size of the notebook personal computer 3, but these members are small. Therefore, the members forming the hinge device 9 of the comparative technique are enlarged and depicted in FIG. 1C.
The hinge device 9 depicted in FIG. 1C includes a first friction plate 57, a second bracket 58, a second friction plate 59, a washer 60, disc springs 61, and a retainer plate 62 which are placed through a small-diameter section 56 of a support shaft 55 in order. To a display attaching portion 64 of a first bracket 63 connected to the support shaft 55, a display section 2 depicted in FIG. 1A is connected. The hinge device 9 is connected to the main body section 1 depicted in FIG. 1A by a main body attaching portion 65 of the second bracket 58. When the disc springs 61 are compressed by the retainer plate 62, the washer 60, the second friction plate 59, the second bracket 58, and the first friction plate 57 are pressurized by the spring force of the disc springs 61 in the direction in which the support shaft 55 is located, and the washer 60, the second friction plate 59, the second bracket 58, and the first friction plate 57 are brought into contact with each other by pressure.
Although the second bracket 58 is rotatable around the small-diameter section 56, the first friction plate 57 and the second friction plate 59 are not rotatable around the small-diameter section 56. As a result, the first friction plate 57 and the second bracket 58 and the second bracket 58 and the second friction plate 59 form a friction plate structure 66. Friction torque T acts on the friction plate structure 66. When the display section 2 is opened and closed with hands, if driving torque Q acting on the support shaft 55 becomes higher than the friction torque T as a result of the opening and closing operation performed by the hands, the support shaft 55 rotates and the display section 2 performs an opening and closing operation. If the hands are released when the display section 2 reaches a certain opening angle φ, the driving torque Q becomes 0 and the driving torque Q becomes lower than the friction torque T, and the display section 2 makes a free stop in a position of the angle φ.
The friction plate structure 66 in the hinge device 9 depicted in FIG. 1C is formed of annular friction plates that rub the flat surfaces against each other, the flat surfaces facing each other. The friction torque in this structure will be described by using a model of the annular friction plates depicted in FIG. 2A. Here, the outside diameter of two annular friction plates 51 and 52 is assumed to be φ1, the inside diameter thereof is φ2, the coefficient of friction of the frictional surfaces of the two annular friction plates 51 and 52 is assumed to be μ, and the friction torque produced when the applied pressure applied to the two annular friction plates 51 and 52 is F is assumed to be T.
Assume that Rw represents an equivalent friction radius. Then, the friction torque T produced when the applied pressure F is applied to the two annular friction plates 51 and 52 is given by formula (1) below, and the equivalent friction radius Rw is given by formula (2) below.T=Rw·μ·F  (1)Rw=(φ13−φ23)/[3·(φ12−φ22)]  (2)
Here, a case in which the equivalent friction radius Rw and the magnitude of the friction torque T are calculated when the inside diameter φ2 is set at 3 mm which is fixed and the outside diameter φ1 is varied: 7 mm, 6 mm, and 5 mm is depicted in tabular form in FIG. 2B. As is clear from this table, the smaller the outside diameter of the friction plate included in the hinge device, the lower the friction torque T.
On the other hand, notebook personal computers are increasingly made thinner and lighter to improve the convenience when the notebook personal computers are carried around. In addition, when the notebook personal computer is made thinner, the outside diameter of a friction plate included in a hinge device has to be reduced. However, as depicted in FIG. 2B, when the outside diameter of the friction plate included in the hinge device is reduced, the friction torque T desired for the notebook personal computer may not be obtained.